Non-volatile programmable variable resistance element
A phase-change memory element exhibits a non-uniform temperature profile in the phase-change material, resulting in a non-uniform temperature profile. The non-uniform temperature profile causes non-uniform growth of a programmed volume, resulting in a gradual R-I characteristic. The phase-change material may be a chalcogenide material.
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The present invention is related to electrically programmable resistance memory. In particular, the present invention relates to phase-change memory.
BACKGROUND OF THE INVENTIONProgrammable resistance memory elements formed from materials that can be programmed to exhibit at least a high or low stable ohmic state are known in the art. Such programmable resistance elements may be programmed to a high resistance state to store, for example, a logic ONE data bit or programmed to a low resistance state to store a logic ZERO data bit.
One type of material that can be used as the memory material for programmable resistance elements is phase-change material. Phase-change materials may be programmed between a first structural state where the material is generally more amorphous (less ordered) and a second structural state where the material is generally more crystalline (more ordered). The term “amorphous”, as used herein, refers to a condition which is relatively structurally less ordered or more disordered than a single crystal and has a detectable characteristic, such as high electrical resistivity. The term “crystalline”, as used herein, refers to a condition which is relatively structurally more ordered than amorphous and has lower electrical resistivity than the amorphous state.
The phase-change materials may be programmed between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. That is, the programming of such materials is not required to take place between completely amorphous and completely crystalline states but rather the material can be programmed in incremental steps reflecting (1) changes of local order, or (2) changes in volume of two or more materials having different local order so as to provide a “gray scale” represented by a multiplicity of conditions of local order spanning the spectrum between the completely amorphous and the completely crystalline states. For example, phase-change materials may be programmed between different resistive states while in crystalline form.
A volume of phase-change material may be programmed between a more ordered, low resistance state and a less ordered, high resistance state. A volume of phase-change is capable of being transformed from a high resistance state to a low resistance state in response to the input of a single pulse of energy referred to as a “SET pulse”. The SET pulse is sufficient to transform the volume of memory material from the high resistance state to the low resistance state. It is believed that application of a SET pulse to the volume of memory material changes the local order of at least a portion of the volume of memory material. Specifically, it is believed that the SET pulse is sufficient to change at least a portion of the volume of memory material from a less-ordered amorphous state to a more-ordered crystalline state.
The volume of memory material is also capable of being transformed from the low resistance state to the high resistance state in response to the input of a single pulse of energy which is referred to as a “RESET pulse”. The RESET pulse is sufficient to transform the volume of memory material from the low resistance state to the high resistance state. While not wishing to be bound by theory, it is believed that application of a RESET pulse to the volume of memory material changes the local order of at least a portion of the volume of memory material. Specifically, it is believed that the RESET pulse is sufficient to change at least a portion of the volume of memory material from a more-ordered crystalline state to a less-ordered amorphous state.
The use of phase-change materials for electronic memory applications is known in the art. Phase-change materials and electrically programmable memory elements formed from such materials are disclosed, for example, in U.S. Pat. Nos. 5,166,758, 5,296,716, 5,414,271, 5,359,205, 5,341,328, 5,536,947, 5,534,712, 5,687,112, and 5,825,046 the disclosures of which are all incorporated by reference herein. Still another example of a phase-change memory element is provided in U.S. patent application Ser. No. 09/276,273, the disclosure of which is also incorporated herein by reference.
It is important to be able to accurately read the resistance states of programmable resistance elements which are arranged in a memory array. The present invention describes an apparatus and method for accurately determining the resistance states of programmable resistance elements arranged as memory cells in a memory array. Background art circuitry is provided in U.S. Pat. No. 4,272,833 which describes a reading apparatus based upon the variation in the threshold levels of memory elements, and U.S. Pat. No. 5,883,827 which describes an apparatus using a fixed resistance element to generate reference signals. Both U.S. Pat. No. 4,272,833 and U.S. Pat. No. 5,883,827 are incorporated by reference herein.
SUMMARY OF THE INVENTIONA phase change memory in accordance with the principles of the present invention includes a volume of phase change material in electrical communication with two contacts and at least one thermal element in thermal communication with the volume of the phase change material. In one aspect of the invention, the thermal element may be situated to increase (relative to a memory without the thermal element) the volume fraction of phase change material that must be amorphized in order to yield a given resistance between the two electrodes.
A phase change memory in accordance with the principles of the present invention may be characterized by the amount of energy required to melt at least a portion of the phase change material proximate the area of a contact and the greater amount of energy required to melt enough of the material for the phase change material to reach a saturation resistance. The melt and saturation resistances are defined herein respectively as 0.5% and 90% of the maximum resistance exhibited by the phase change device. The current pulses corresponding to these energies are referred to herein, respectively, as IMELT and ISAT. In accordance with the principles of the present invention, a thermal element in thermal communication with the phase change material increases the difference between the energy corresponding to IMELT and the energy corresponding to ISAT while limiting IMELT Limiting, that is, any increase in the energy required to melt at least a portion of phase change material proximate to a contact.
In another aspect of a phase change memory in accordance with the principles of the present invention, a thermal element is placed in thermal communication with phase change material in a manner that discourages amorphization of a least resistance path cross section of the phase change material. As employed herein, the term least resistance path cross section refers to a cross section of the path of least electrical resistance from one electrode to another across the phase change material. The area of this critical cross section is less than a cross section of the entire phase change volume but at least as great as the minimal cross section required to carry a current sufficient to drive the phase change material to saturation using a predetermined current pulse-width.
One or more thermal elements are employed in a phase change memory in accordance with the principles of the present invention to increase the difference in amplitude between current pulses within the IMELT to Isat range for corresponding increases in device resistances. By increasing differences in programming current within this range, current/resistance pairs may be more readily distinguishable, thereby enhancing the ability to store multiple levels of information within a single phase change memory element.
In illustrative embodiments, the thermal elements may be dissipating or conserving elements, acting respectively, as heat sinks or as thermal insulation. In such embodiments, the thermal element(s) is arranged to induce a thermal profile within the phase change material that includes one or more thermal gradients. The thermal elements may be arranged to induce gradients that increase the volume percentage of phase change material that is amorphized for a given resistance change in the device's RESET current regime.
FIGS. 5A through 5F′ show non-uniform temperature profile about the area of contact for a phase-change memory element.
At the leftmost extreme of the graph the resistance is a relatively high 1 MΩ, characteristic of a RESET condition for the phase change device in which it was left. The device could also have been left in a SET state. A current pulse of amplitude ISET crystallizes a sufficient amount of phase change material within the phase change device to provide a relatively low resistance path through the phase change material from one electrode to another. Shorter programming current pulses may be applied to the phase change device in an attempt to increase the resistance of the device. In the illustrative embodiment of
At the amplitude of IMELT, the energy content of a current pulse is sufficient to melt at least a portion of phase change material proximate the area of contact between the phase change material and an electrode. Because the relatively short duration of a programming current pulse (50 nanoseconds in this illustrative embodiment) allows the melted material to cool before significant crystallization of the melted material may occur, a portion of phase change material in the current path between the electrodes is transformed by the current pulse IMELT to an amorphous state characterized by a relatively high electrical resistance. The value of increased resistance selected to define IMELT may be somewhat arbitrary. For the sake of this discussion, a resistance increase from the low, SET, resistance, to 0.5% of the high, RESET, resistance corresponds to the melt resistance, RMELT, and melt current, IMELT. At the other extreme of amorphization, the saturation resistance is chosen as 90% of the RESET resistance and saturation current, ISAT, the corresponding current pulse amplitude.
As can be seen from the graph of
This narrow range of programming pulse amplitudes does not pose a problem for binary operation. Even with wide variation in device-to-device characteristics, the differences in programming current and resultant resistance values are relatively easy to distinguish. However, for multi-level operation in which more than two states, corresponding to more than one bit of information, may be stored within a single phase-change cell, the relatively narrow current pulse amplitude window does pose a problem.
That is, if, in order to increase the information storage density of the phase change device, the range of programming currents and resultant resistances are divided into sub-ranges, with current amplitudes intermediate to IMELT and ISAT assigned logic values (for example, R1 and R2, assigned logic values two and three, respectively), device variations could easily cause a slight shift in current amplitude response that could lead to an errant resistance reading for a given programming pulse. The resistance resulting from a programming pulse I1 may be misread as the resistance resulting from a programming pulse IMELT, for example.
In order to increase the error margin and thereby make multi-level storage more practical, even with somewhat relaxed manufacturing margins, it is essential to extend the range of current amplitudes, IMELT to Isat, corresponding to the range of resistance values, RMELT-Rsat. Ideally, one would extend this range while minimizing any increase in total energy required to program the phase change device.
The RI graph of
For the purpose of illustration, the cross-sectional view of
Another way to visualize the transformation process is to realize that all the energy delivered to the volume 200 in the form of programming pulse IMELT has gone into transforming the minimal volume of phase change material, VMELT, to yield a resistance RMELT. In a real device the pictorial layers represent “volume fractions” of the phase change alloy's total volume. And in reality, any non-uniformity in the cross section, any “bump” in the layer LMELT, would necessarily include an area of lower resistance, through which current between the electrodes 202 and 204 would preferentially flow: a path of least resistance.
Similarly, current pulses I1, I2 of greater amplitude would yield volume fraction changes, symbolized by layers L1, L2 of amorphized phase change material with corresponding resistances R1, R2. Each amorphization step for each programming current amplitude increase, is the minimal increase resulting from the amorphization of additional phase change material. Reaching a resistance level Rsat, corresponding to the application of a programming current Isat, would therefore require the minimal increase in energy input and the difference in amplitude between IMELT and Isat would be minimal.
Recognizing that greater energy differences and, concomitantly, greater differences in programming current amplitudes, may be associated with imposing a non-uniformity on the minimal energy amorphization layers (e.g. LMELT, L1, L2, LRESET) depicted in
By manipulating the thermal environment of a phase change memory in accordance with the principles of the present invention, phase change memories exhibit a broader range of programming current values between IMELT and Isat, thereby easing the process of discriminating the various resistance levels associated with a multi-level phase-change memory.
At the same time, a great deal of effort has been expended in attempts at reducing the energy required to program a phase change memory. A phase change memory in accordance with the principles of the present invention manipulates the thermal environment of the phase change volume 200 to minimize the total energy (current amplitude) required to program the device. A phase change memory in accordance with the principles of the present invention employs one or more thermal elements to increase the difference between IMELT and Isat while, at the same time, minimizing both IMELT and Isat. This process may be visualized as a “flattening” of the I/R curve from IMELT to Isat in which IMELT is substantially unchanged and Isat is increased to improve the discrimination of memory states.
A phase change memory in accordance with the principles of the present invention includes a volume of phase change material in electrical communication with two contacts and at least one thermal element in thermal communication with the volume of the phase change material. The thermal element is situated to increase the volume fraction of phase change material that must be amorphized in order to yield a given resistance between the two electrodes. The cross sectional diagram of
In this illustrative embodiment, just as in the minimal energy case described in the discussion related to
In this conceptual representation, Vsat is the maximum volume of phase change material that can be amorphized to yield a value of Rsat between the electrodes 202 and 204. In this illustrative embodiment, the increase in programmed volume for each intermediate value of resistance is substantially equal, so that, for example, the difference between V1 and VMELT is substantially equal to the difference between V1 and V2, which is substantially equal to the difference between V2 and VRESET. Additionally, the increases in programmed volumes in the path of least resistance are substantially equal, so that the difference between VMELTLR and V1LR is approximately equal to the difference between V1LR and V2LR and between V2LR and VRESETLR. Because substantially more phase change material is amorphized (VADD, above the broken line 206) to reach Rsat, the corresponding current amplitude Isat is also increased.
Because the volume differences are substantially equal, the spacings between the corresponding resistances are also substantially equal. As will be described in greater detail in the discussion below, non-uniformities in the thermal profile of the phase change volume 200, established by thermal elements in accordance with the principles of the present invention, yield volume amorphization non-uniformities in the phase change material to increase the difference between IMELT and IRESET for a volume of phase change material.
In the illustrative embodiment of
With one or more thermal elements placed in thermal communication with a device's phase change material in a manner that discourages amorphization of a portion of the phase change material, the thermal element(s) delays the formation of a high-resistance cross-section between the device's electrodes, thereby increasing the range of current amplitudes between IMELT and Isat. By increasing differences in programming current within this range, current/resistance pairs may be more readily distinguishable, thereby enhancing the ability to store multiple levels of information within a single phase change memory element.
As illustrated in the conceptual diagrams of
In the phase change memory of
When a current pulse is applied to the memory device 300 energy is concentrated in the region around the interface between the heater 303 and phase change material 306. Consequently, phase change material is amorphized in a pattern that somewhat resembles the head of a mushroom. Current pulses of different amplitudes I1, I2, I3 . . . yield “mushroom heads” of different volumes V1, V2, V3, . . . . Eventually, the “mushroom head” of amorphized material reaches a point where the path of least resistance between the top 310 and bottom 302 electrodes, across the programmed volume 312, exhibits a resistance substantially equal to the reset resistance of the device.
In the somewhat stylized representation of
By altering the thermal profile of a phase change memory in accordance with the principles of the present invention, we may extend the range of current pulse amplitudes falling between IMELT and IRESET, thereby providing for improved discrimination of intermediate memory states and allowing higher-density memory storage.
In the illustrative embodiment of
In an illustrative embodiment a relatively heavy conductor, such as a top contact, may serve double duty as both an electrical channel for accessing the memory device 314 through the electrode 326 and as a heat sink 328 in thermal communication with the device. The somewhat stylized view of the progress of amorphization in
In this illustrative embodiment, as programming amplitudes are increased from IMELT to I1, I2, I3, instead of obtaining the familiar mushroom head amorphization volume illustrated in
Current will, of course, flow through the area of least resistance and the device resistance will be that exhibited by the area of least resistance. Because the heat sink will have little effect on the amorphization of material proximate the heater 318, the current pulse amplitude corresponding to IMELT should be substantially equal to that of a similar device without the heat sink 328. However, the distortions of the programmed volume attendant the addition of the heat sink 328 will increase the value of Isat (and of currents intermediate to Isat), thereby extending the current range for corresponding values of RMELT and Rsat and affording greater discrimination of intermediate memory levels.
One or more additional areas or thicknesses of insulator material may be added to the device to further reduce overall current requirements for the device, to induce a thermal gradient upon a device's phase change material, or to impose a thermal gradient in cooperation with a heat sink. The insulator 330 of
A second conductive layer 450 is deposited over the phase-change memory material 440. The second conductive layer serves as a second electrode (or second electrical contact) for the memory element. The first electrode 420 and the phase-change material 440 form a first area of contact 435 through which the first electrode 420 and the phase-change material electrically communicate.
At least a portion of the phase-change material situated in the proximity of the first area of contact 435 is active and switches between an amorphous and a crystalline phase. In order to control the resistance of the memory element, the amount of phase-change material that actually changes phase may be controlled.
Increased control over the volume fraction of amorphous to crystalline material may be accomplished by designing a memory device structure (the shapes, sizes and compositions of materials as well as the structural relationship between the components) which is effective to provide non-uniform heating of the phase-change material about the area of contact. Such non-uniform heating creates a non-uniform temperature profile in the phase-change material about the area of contact 435. Non-uniform heating of the phase-change material results in non-uniform crystallization of the phase-change material about the area of contact so that the volume percentage of amorphous material to crystalline material can be more accurately controlled.
Generally, any non-uniform temperature profile may be used in the present invention. One example of a non-uniform temperature profile is shown in
Hence,
FIG. 5A′ shows a top view of the phase-change material about the area of contact 435. FIG. 5A′ shows that from radial distance 0 to r1, the active phase-change material is in a substantially crystalline state as represented by crystalline region C1. Likewise, FIG. 5A′ shows that from radial distance r1 to R, the active phase-change material at the area of contact 435 is in a substantially amorphous state as represented by amorphous region A1. Hence, it is seen that in the active phase-change material at the area of contact 435, a volume fraction of the phase-change material is a crystalline region C1 and a volume fraction is an amorphous region A1. The resistance state of the memory element may be represented by a resistance state RA.
FIG. 5B′ shows a top view of the phase-change material at the area of contact 435. FIG. 5B′ shows that from radial distance 0 to r2 (where r2 is less than r1), the phase-change material at the area of contact 435 is in a substantially crystalline state as represented by crystalline region C2. Likewise, FIG. 5B′ shows that from radial distance r2 to R, the phase-change material at the area of contact 435 is in a substantially amorphous state as represented by amorphous region A2. The phase-change material at the area of contact 435 has a volume fraction in a crystalline region C2 and a volume fraction in an amorphous region A2. The resistance state of the memory element may be represented by a resistance state RB. The resistance RB is greater than the resistance RA.
FIG. 5C′ shows a top view of the phase-change material at the area of contact 435. FIG. 5C′ shows that from 0 to distance r3 (which is less than r2), the active phase-change material is in a substantially crystalline state as represented by crystalline region C3. Likewise, FIG. 5C′ shows that from radial distance r3 to R, the active phase-change material at the area of contact is in a substantially amorphous state as represented by amorphous region A3. The active phase-change material at the area of contact 435 has a volume fraction of the phase-change material is a crystalline region C3 and a volume fraction is an amorphous region A3. The resistance state of the memory element may be represented by a resistance state RC. The resistance of RC is greater than that of the resistance state RB.
The temperature profiles TA, TB and TC shown in
As an example,
One way to stably control the resistance of the phase-change memory element, is to control the amount of material changing phase. In a planar device, such as the devices shown in FIGS. 4A and 4B, this can be controlled by the vertical heat loss through the upper electrode 450 and the lower electrode 420. It is noted that these electrode may be thermally resistive enough to provide sufficient heating of the phase-change material to provide low power. However, may also be thermally conductive enough so that the vertical heat conduction leads to a substantial non-uniformity between the center and edge of the area of contact 435 of the device.
The thermal conductivity of the electrodes 450, 420 may be varied, for example, by varying the resistivity of the electrode material and/or the dimensions (e.g. the thicknesses) of the electrode material. The thicknesses of one or both of the electrodes may be varied to provide improved multi-state operation. For example, while not wishing to be bound by theory, it is believed that a thicker top electrode and/or bottom electrode creates greater heat sinking which may be better for multi-state operation since it may create less uniform heating in the phase-change material about the area of contact. In an embodiment of the invention, the thickness of the conductive layer 450 (e.g. the top electrode) may be made thicker than the thickness of the phase-change layer 440. In another embodiment of the invention, the thickness of the conductive layer 420 (e.g. the bottom electrode) may be made thicker than the thickness of the phase-change layer 440.
Likewise, varying the width of the opening 432, thereby creating a larger area of contact 435 between the bottom electrode 420 and the phase-change material also provides for increased non-uniformity of the heating of the phase-change material about the area of contact 435. As noted above, in one embodiment of the invention, the width of the opening 432 may be made to greater than or equal to about (⅔)F where F is the feature size of the device. In another embodiment of the invention, the width of the opening 432 may be greater than the feature size F.
In the example shown in
In the example shown in
The device shown in
FIG. 5D′ shows a top view of the phase-change material about the area of contact 535. FIG. 5D′ shows that from 0 to radius r4, the active phase-change material is in a substantially amorphous state as represented by crystalline region A4. Likewise, FIG. 5D′ shows that from radius r4 to R, the active phase-change material about the area of contact is in a substantially crystalline state as represented by crystalline region C4. Hence, it is seen that in the active phase-change material about the area of contact 535, a volume fraction of the phase-change material is a crystalline region C4 and a volume fraction is an amorphous region A4. The resistance state of the memory element may be represented by a resistance state RSD.
FIG. 5E′ shows a top view of the phase-change material about the area of contact 535. FIG. 5E′ shows that from 0 to radius r5 (which is greater than r4), the active phase-change material is in a substantially amorphous state as represented by amorphous region A5. Likewise, FIG. 5E′ shows that from radius r5 to R, the active phase-change material about the area of contact is in a substantially crystalline state as represented by crystalline region C5. The active phase-change material about the area of contact 535 has a volume fraction of the phase-change material in an amorphous region A5 and a crystalline region C5 and a state of the memory element may be represented by a resistance state RE. The resistance RE is greater than the resistance RD.
FIG. 5F′ shows a top view of the phase-change material about the area of contact 435. FIG. 5F′ shows that from 0 to radius r6 (which is greater than r5), the active phase-change material is in a substantially amorphous state as represented by the amorphous region A6. Likewise, FIG. 6F′ shows that from radius r6 to R, the active phase-change material about the area of contact is in a substantially crystalline state as represented by crystalline region C6. The active phase-change material about the area of contact 435 has a volume fraction of the phase-change material is a crystalline region C6 and a volume fraction is an amorphous region A6. The resistance state of the memory element may be represented by a resistance state RF. The resistance of RF is greater than that of the resistance state RE.
The temperature profiles T4, T5 and T6 shown in
Another way to provide for a non-uniform temperature profile in the phase-change material about the area of contact is to increase the aspect ratio (e.g. length to width) of the area of contact between the electrode and the active phase-change material.
Changing the structure of the memory element as well as the characteristics of the memory element components (e.g. thickness of the top electrode, resistivity of the top electrode, thickness of the phase-change material, size of the area of contact, aspect ratio of the area of contact, etc) changes the shape of the I-R curve shown in
Referring to
The current amplitude IMELT is the amplitude of a programming current pulse which is the minimum current that is sufficient to cause the melting of at least a portion of the chalcogenide material about the area of contact. It is seen that as the amplitude of the programming current pulse continues to increase above IMELT, the programmed resistance along the right side of the I-R curve continues to rise. The rise is substantially linear until saturation is reached at which point, the slope of the I-R curve begins to decrease. As shown in
In an illustrative embodiment, the ratio of ISAT to IMELT is about 1.5 to 1. However, the ratio of ISAT to IMELT may be increased by increasing the non-uniformity of the temperature profile about the area of contact. Hence, in one embodiment of the invention, the ratio of ISAT to IMELT may be at least 3 to 1. In another embodiment, the ratio of ISAT to IMELT may at least 4 to 1. In yet another embodiment, the ratio of ISAT to IMELT may be at least 6 to 1. As seen, increasing the ratio of ISAT to IMELT, flattens (e.g. lessens the slope) of the right side of the I-R curve so as to provide for improved multi-state operation.
There are yet other ways to improve the multi-state behavior of a phase-change memory element. This is to provide a thermal inhomogeniety in the vicinity of the area of contact, causing non-uniform heat flow. For example a memory element similar to that shown in
The thermal conductivity of the conductive layer behaves as a heat sink so as to draw away heat from the active region of the phase-change material. Hence, the heat flow within the bulk of the phase-change material is uneven or asymmetric such that there is more heat flow from the phase-change material to the conductive material and less heat flow from the phase-change material to the insulative material. Hence, the region of the phase-change material in the proximity of the conductive material becomes cooler at a faster rate so as to provide a region of increased quenching of the phase-change material. This region of increased quenching results in an asymmetric region of amorphous material within the region of phase-change material. While not wishing to be bound by theory, it is believed that the asymmetry of this region of amorphous material results in flattening the right side of the I-R curve shown in
Examples of the materials which may be used for top and bottom electrodes described herein include, but are not limited to, titanium nitride, titanium aluminum nitride, titanium carbonitride, titanium silicon nitride, molybdenum, carbon, tungsten, tungsten silicide, titanium-tungsten, n-type doped polysilicon, p-type doped polysilicon, n-type doped silicon carbon compounds and/or alloys, p-type doped silicon carbon compounds and/or alloy.
Examples of materials which may be used as the dielectric or insulative layers include oxides and nitrides. Examples of oxide include silicon oxide. Examples of nitrides include silicon nitride.
The memory material may be a phase-change material. The phase-change materials may be any phase-change memory material known in the art. The phase-change materials may be capable of exhibiting a first order phase transition. Examples of materials are described in U.S. Pat. Nos. 5,166,758, 5,296,716, 5,414,271, 5,359,205, 5,341,328, 5,536,947, 5,534,712, 5,687,112, and 5,825,046 the disclosures of which are all incorporated by reference herein.
The phase-change materials may be formed from a plurality of atomic elements. Preferably, the memory material includes at least one chalcogen element. The chalcogen element may be chosen from the group consisting of Te, Se, and mixtures or alloys thereof. The memory material may further include at least one element selected from the group consisting of Ge, Sb, Bi, Pb, Sn, As, S, Si, P, O, and mixtures or alloys thereof. In one embodiment, the memory material comprises the elements Te, Ge and Sb. In another embodiment, the memory material consists essentially of Ge, Sb and Te. An example of a memory material which may be used is Ge2Sb2Te.
The memory material may include at least one transition metal element. The term “transition metal” as used herein includes elements 21 to 30, 39 to 48, 57 and 72 to 80. Preferably, the one or more transition metal elements are selected from the group consisting of Cr, Fe, Ni, Nb, Pd, Pt and mixtures or alloys thereof. The memory materials which include transition metals may be elementally modified forms of the memory materials in the Te—Ge—Sb ternary system. This elemental modification may be achieved by the incorporation of transition metals into the basic Te—Ge—Sb ternary system, with or without an additional chalcogen element, such as Se.
It is to be understood that the disclosure set forth herein is presented in the form of detailed embodiments described for the purpose of making a full and complete disclosure of the present invention, and that such details are not to be interpreted as limiting the true scope of this invention as set forth and defined in the appended claims.
Claims
1. A phase-change memory, comprising:
- a volume of phase change material; and
- a thermal element in thermal communication with the volume of phase change material, the thermal element configured to preferentially shape the volume portion of phase change material that is amorphized in response to the passing of a current pulse through the phase change material.
2. The phase-change memory of claim 1 wherein the thermal element is configured to impose a thermal non-uniformity upon the phase change material.
3. The phase-change memory of claim 2 wherein the thermal non-uniformity is configured to increase the volume fraction of phase change material amorphized for a given increase in the amplitude of a programming current without decreasing the RESET current of the device.
4. The phase-change memory of claim 1 wherein the thermal element is configured to direct energy to preferentially amorphize a volume of phase change material that is not determinative of the phase change memory's RESET current.
5. The phase-change memory of claim 1 wherein the thermal element is configured to retard amorphization along a path of least resistance within the volume of phase change material.
6. The phase change memory of claim 1 wherein the thermal element operates as a heat dissipating device.
7. The phase change memory of claim 1 wherein the thermal element operates as a heat conserving device.
8. The phase change memory of claim 6 wherein the thermal element is an electrode.
9. The phase change memory of claim 6 wherein the thermal element is an address line.
10. The phase change memory of claim 7 wherein the thermal element is device insulation.
11. The phase change memory of claim 6 wherein the thermal element is an absence of insulation.
12. The phase change memory of claim 1 comprising a plurality of thermal elements in thermal communication with the phase change material.
13. The phase change memory of claim 12 wherein at least one thermal element is heat dissipative.
14. The phase change memory of claim 12 wherein at least one thermal element is heat conservative.
15. The phase change memory of claim 1 further comprising a heater.
16. The phase change memory of claim 1 wherein the thermal element imposes a lateral thermal non-uniformity on the phase change material.
17. The phase change memory of claim 1 wherein the thermal element imposes a vertical thermal non-uniformity on the phase change material.
18. A phase change memory comprising:
- a phase-change material; and
- an electrode in electrical communication with said phase-change material, said phase-change material and said electrode having an area of contact, said memory element being adapted so that application of an energy to said memory element creates a non-uniform temperature profile in the phase-change material about said area of contact.
19. The memory of claim 18, wherein said area of contact has a periphery, said temperature profile being characterized by a first temperature proximate said periphery and a second temperature remote said periphery, said first temperature being greater than said second temperature.
20. The memory element of claim 18, wherein said area of contact has a periphery, said temperature profile being characterized by a first temperature proximate said periphery and a second temperature remote said periphery, said first temperature being less than said second temperature.
21. The memory element of claim 18, wherein said phase-change material comprises a chalcogen element.
22. A memory element, comprising:
- a phase-change material;
- a heat source in thermal communication with said phase-change material, said heat source providing thermal energy to said phase-change material; and
- a heat dissipative element in thermal communication with said phase-change material, said heat dissipative element configured to remove thermal energy from said phase-change material, said heat dissipative element being asymmetrically positioned relative to said heat source.
23. The memory element of claim 22, where there is asymmetric heat flow between said heat source and said heat dissipative element.
24. The memory element of claim 22, wherein said heat flow is asymmetric relative to a vertical plane about the heat source.
Type: Application
Filed: Jan 2, 2008
Publication Date: Jul 2, 2009
Applicant:
Inventors: Wolodymyr Czubatyj (Warren, MI), Guy Wicker (Southfield, MI)
Application Number: 12/006,390
International Classification: H01L 45/00 (20060101);